1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine.
2. Description of the Related Art
A conventional spark plug generally includes a center electrode projecting downward from the tip face of an insulator, and a parallel ground electrode disposed in opposition to the center electrode while one end of the ground electrode is joined to a metallic shell. The spark plug is adapted to ignite an air-fuel mixture by means of spark discharge effected across an air gap between the center electrode and the parallel ground electrode. In addition to such a parallel-electrode spark plug, a creeping-discharge spark plug is known which is a spark plug for use in an internal combustion engine and which features improved fouling resistivity. The creeping-discharge spark plug is configured such that sparks produced in a spark discharge gap creep along the surface of an insulator in the form of creeping discharge at all times or under certain conditions.
For example, a so-called semi-creeping-discharge spark plug includes an insulator having a center through-hole formed therein; a center electrode held in the center through-hole and disposed at a tip portion of the insulator; a metallic shell for holding the insulator such that a tip portion of the insulator projects from the tip face thereof; and a semi-creepage ground electrode disposed such that one end thereof is joined to the metallic shell while the other end thereof faces either the side peripheral surface of the center electrode or the side peripheral surface of the insulator. Creeping discharge involves air discharge effected between the spark face of the semi-creepage ground electrode and the surface of the insulator and sparking that creeps along the tip surface of the insulator. In the spark plug of creeping discharge type, spark discharge occurs so as to creep along the surface of the insulator, thereby continuously burning off fouling and thus exhibiting enhanced fouling resistivity as compared with a spark plug of air discharge.
A hybrid spark plug has been proposed which combines functions of the parallel-electrode type spark plug and the semi-creeping-discharge type spark plug. Since dimensions of the hybrid spark plug are determined such that sparking occurs across a semi-creepage gap even when the tip face of an insulator is not fouled, channeling can be effectively suppressed while fouling resistivity is established, and ignition property can be improved.
Among hybrid spark plugs composed of a parallel ground electrode and a semi-creepage ground electrode, a certain hybrid spark plug includes a heat release acceleration metal portion provided in a center electrode in order to accelerate heat release from the center electrode, the heat release acceleration metal portion being made of a material higher in heat conduction than an electrode base material. As shown in FIG. 10, the heat release acceleration metal portion 2m is provided in the interior of the electrode base material so as to accelerate heat release from the entire center electrode, thereby effecting good heat release from the center electrode. The larger the portion of the electrode base material occupied by the heat release acceleration metal, the greater the heat release effect.
3. Problems Solved by the Invention
However, for structural reasons, increasing a portion of the center electrode occupied by the heat release acceleration metal portion unavoidably involves a reduction in the wall thickness of the electrode base material. This potentially results in impaired durability against surface erosion of the electrode base material stemming from spark discharge across a semi-creepage gap.
The hybrid spark plug potentially involves a variation over the course of time in the frequency of sparking across a certain gap depending on engine conditions, engine characteristics, and the like. Dimensions of the hybrid spark plug are determined such that sparking across the semi-creepage gap occurs, even when carbon fouling does not occur as well as when carbon fouling occurs. In the case of such a spark plug involving highly frequent sparking against the side surface of a center electrode, a problem of spark erosion of the side surface of the center electrode arises.
An object of the present invention is to provide a hybrid spark plug including a parallel ground electrode and a semi-creepage ground electrode, which spark plug exhibits good heat release from a center electrode and excellent durability against spark erosion by effectively protecting a portion of the side peripheral surface of the center electrode subjected to frequent spark impact.
To achieve the above object, the present invention provides a spark plug comprising:
an insulator having a center through-hole formed therein; a center electrode held in the center through-hole, disposed in a tip portion of the insulator, and having a noble metal chip located at a tip portion thereof, a metallic shell for holding the insulator such that a tip portion of the insulator projects from a tip face thereof, a parallel ground electrode disposed such that one end thereof is joined to the tip face of the metallic shell while the other end thereof faces a tip face of the center electrode so as to form a main air gap; and a plurality of semi-creepage ground electrodes each disposed such that one end thereof is joined to the metallic shell while the other end thereof faces at least either the side peripheral surface of the center electrode or the side peripheral surface of the insulator so as to form a semi-creepage gap.
The spark plug is characterized in that a tip portion of the center electrode as projected orthogonally on a virtual plane in parallel with the axis of the center electrode includes a tapered portion which is tapered such that its diameter reduces axially frontward, where the term frontward refers to an axial direction directed into an internal combustion engine; a convex portion is formed at an axially intermediate position of the tapered portion such that an outline thereof as viewed on the virtual plane projects radially outward with respect to the axis; the axially measured distance between the vertex of the convex portion (hereinafter may be called the convex vertex) and the tip of the insulator is less than 0.5 mm; a heat release acceleration metal portion higher in thermal conductivity and linear expansion coefficient than an electrode base material, which forms a surface layer portion of the center electrode, is present at a position located 1.5 mm axially rearward from the convex vertex while being enclosed by the electrode base material; and the heat release acceleration metal portion is formed such that the electrode base material has a wall thickness of not less than 0.6 mm as measured at a position located 1.5 mm axially rearward from the convex vertex.
As described above, the center electrode has a convex portion formed such that the axially measured distance between the convex vertex and the tip face of the insulator is less than 0.5 mm, thereby yielding the following effect: sparks which creep along the tip surface of the insulator can readily reach the convex vertex, which is angular and on which an electric field concentrates, thereby maintaining good ignition property at a gap between the semi-creepage ground electrode and the center electrode. Since sparks generated between the electrodes creep along the tip face of the insulator, the sparks erode, for example, a portion of the center electrode located rearward of the convex vertex, such as the region C in FIG. 10.
Thus, by employing the above-described configuration in which the heat release acceleration metal portion is present at a position located 1.5 mm axially rearward from the vertex of the convex portion of the center electrode having the noble metal chip located at the tip portion, the heat release acceleration metal portion suppresses an increase in electrode temperature. Additionally, by imparting to the electrode base material a wall thickness of not less than 0.6 mm as measured at a position located 1.5 mm axially rearward from the convex vertex, the electrode base material becomes sufficiently thick to withstand progress of erosion associated with spark discharge across a semi-creepage gap, thereby contributing to maintenance of spark plug performance over a long period of time. The heat release acceleration metal portion is higher in thermal conductivity and linear expansion coefficient than the electrode base material. Such a combination of the electrode base material and the heat release acceleration metal portion, which are made of different materials, potentially involves a burst phenomenon in which, when the electrode base material becomes thin as a result of progress of erosion, the difference in thermal shrinkage causes the heat acceleration metal portion to burst out of the electrode base metal before being exposed as a result of erosion. The burst phenomenon can be prevented, as mentioned above, by imparting a sufficient wall thickness to a portion of the electrode base material which is potentially eroded.
In addition to the above-described configuration, the heat release acceleration metal portion may be formed within the center electrode at a position located less than 1.5 mm as measured axially from the tip of the electrode base material located on the spark gap side. As compared to the case of the prior art configuration shown in FIG. 10, such frontward extension of the heat release acceleration metal portion allows an increase in the wall thickness of the electrode base material while the percentage of the heat release acceleration metal portion to the center electrode is held unchanged. Also, the heat release acceleration metal portion is disposed throughout the center electrode, thereby effectively enhancing heat release from the entire center electrode.
Preferably, the above-described spark plug employs the following structural features: a spark erosion resistant metal portion formed of a metal higher in spark erosion resistivity than the electrode base material is formed on the surface of the center electrode in opposition to the semi-creepage ground electrodes; and the axially rearward end of the spark erosion resistant metal portion is located axially frontward of the position located 1.5 mm axially rearward from the convex vertex.
The spark erosion resistant metal portion disposed at a portion of the surface of the center electrode which faces the semi-creepage ground electrode and is potentially eroded by sparks effectively suppresses spark erosion of the surface portion, whereby the spark plug exhibits excellent durability.
In this case, preferably, the spark erosion resistant metal portion formed of a metal higher in spark erosion resistivity than the electrode base material is formed at a portion of the surface of the center electrode which faces the semi-creepage ground electrode and is located axially rearward of the convex vertex; i.e., is located so as not to extend across the convex vertex.
The spark erosion resistant metal portion is disposed so as not to extend across the convex vertex such that the electrode base material which contains a component to suppress spark discharge erosion of the insulator extends across the convex vertex; i.e., such that the electrode base material forms the convex portion. By employing this configuration, a portion of the center electrode located axially rearward of the convex portion is protected by means of the spark erosion resistant metal portion, while in the vicinity of the convex portion sparks collide against the base material of the center electrode, so that the base material of the center electrode scatters. The thus-scattered erosion suppression component contained in the base material of the center electrode adheres to the tip of the insulator. Accordingly, this configuration provides a synergistic effect in that spark erosion of the side peripheral surface of the center electrode is suppressed while channeling is suppressed.
Specifically, for example, the spark erosion resistant metal portion is preferably formed such that the axially frontward end thereof is located axially frontward of a position located 0.5 mm axially rearward from the tip of the insulator. If the spark erosion resistant metal portion is disposed such that the axially frontward end thereof is located axially rearward of the above position, the spark erosion resistant metal portion deviates greatly from a position which is likely to be exposed to sparks, thus failing to yield the effect of suppressing spark erosion of the electrode.
In the above-described spark plug, the insulator may be radiused or chamfered at the opening edge of the center through-hole on the tip face thereof. When the convex vertex is located axially rearward of the tip of the insulator, at the time of semi-creeping discharge, sparks are generated between the semi-creepage ground electrode and the convex vertex via the opening edge of the center through-hole. If the opening edge is not radiused or chamfered, sparks generated via the opening edge cause channeling. Once channeling occurs, spark generation concentrates at a position where channeling occurs; as a result, the intensity of channeling tends to increase. Radiusing or chamfering the opening edge effectively suppresses occurrence of channeling. Preferably, radiusing or chamfering is performed at a radius of curvature of or at a width of 0.05 mm to 0.4 mm.